The present invention relates to a method for pulling single crystals.
The Czochralski method is known, in which a single crystal such as a semiconductor crystal is pulled from molten liquid within a crucible assembly, and is thereby grown. In this case, the crucible assembly is often double and comprises outer and inner crucibles which communicate with each other. When the single crystal is pulled from this double crucible assembly, the crystal is pulled from the surface of the molten liquid or melt within the inner crucible, while a raw material and a dopant are supplied to the outer crucible to compensate for the decrease in volume in the inner crucible, making it possible to pull the single crystal continuously. In this case, the liquid within both the crucibles flows through communication holes in the inner crucible. Accordingly, temperature fluctuations and vibration resulting from the supply of the raw material are not transmitted directly to the surface from which the single crystals is pulled. Thus, there are the advantages that it is possible to obtain stable operation and superior quality.
In general, the dopant concentration of the pulled single crystal is given by kC when the dopant concentration in the molten raw material is C and where k is a segregation coefficient which is usually less than 1. Accordingly, if the molten raw material is not replenished following the pulling, the dopant in the molten raw material becomes gradually more concentrated, and the dopant in the resulting single crystal also becomes more concentrated, as shown in FIG. 1. When the double crucible assembly described above is employed to pull the single crystal continuously, however, the dopant concentration in the raw material fed successively is made equal to the dopant concentration in the pulled single crystals, and the amount of single crystal pulled per unit time is made equal to the amount of supply of the raw material. By doing so, the amounts of dopant supplied and pulled are balanced with each other so that the concentration ratio in dopant between the inner and outer crucibles can always be maintained at C : kC, as shown in FIG. 2. Thus, concentration of the dopant in the molten raw material is prevented, making it possible to render the dopant concentration constant. Accordingly, it is made possible to manufacture a single crystal having no concentration variation in the growth direction.
When the above-described apparatus is employed, however, the dopant concentration is increased in the portion first drawn from the starting material, resulting in a considerable portion of the single crystal being defective. More specifically, when the double crucible assembly is used to pull the single crystal continuously, the supply of the raw material per unit time into the outer crucible is made equal to the amount of the single crystal pulled per unit time to prevent the total amount of the molten liquid from varying. By doing so, however, even if the concentration of the dopant at the initial stage of the pulling is C in both the inner and outer crucibles and the dopant concentration in the subsequently supplied raw material is kC, it is known that concentration occurs in the dopant in the inner crucible at the initial stage of the pulling, resulting in a "hump" in the concentration gradient in the longitudinal direction of the product, as shown in FIG. 3. This "hump" extends for a considerable length along the straight trunk of the crystal, resulting in a large amount of the crystal product having an inappropriate dopant concentration.